1. Field of the Invention
The present invention relates to a dry-type rubber pressing method and apparatus for forming powder into a product under hydrostatic pressure during a dry-type process while squeezing air in the powder into an area which does not adversely affect the formed product during the pressing process.
2. Prior Art
FIG. 41 of the accompanying drawings illustrates a conventional dry-type hydrostatic rubber pressing apparatus. The apparatus includes a mold 2 made of a flexible material such as neoprene rubber, urethane resin, or the like and defining therein an axially elongate powder filling space 1, a pair of lids 3, 4 covering the upper and lower open ends 1a, 1b of the powder filling space 1, a flexible pressure tube 5 fitted over the mold 2, and a retainer case 6 fitted over the pressure tube 5, with an annular pressure chamber 8 defined between the pressure tube 5 and the retainer case 6. The retainer case 6 has liquid supply and discharge ports 6c, 6d opening into the pressure chamber 8. The retainer case 6 has an axial through hole 6a extending vertically as shown, and the pressure tube 5 is disposed in the hole 6a. A core 9 is detachably disposed between the upper and lower lids 3, 4 and extends vertically and axially in the powder filling space 1. The core 9 has upper and lower bolt portions 9a, 9b extending through the lids 3, 4, respectively, with nuts 10, 11 tightened over the bolt portions 9a, 9b, respectively. The mold 2 covered with the upper and lower lids 3, 4 is inserted through the hole 6a into the pressure tube 5, and is retained in place by clamps 12, 13 threaded, respectively in upper and lower openings 6e, 6f defined in the retainer case 6.
The operation of the conventional dry-type rubber pressing apparatus thus constructed will be described below. The upper clamp 12 is detached from the retainer case 6, and the mold 2 is taken out of the retainer case 6. The upper nut 10 and the lid 3 are then removed from the mold 2, and the powder filling space 1 is filled with a mass of powder 14. Thereafter, the upper end of the powder filling space 1 filled with the powder 14 is closed by the lid 3, and the nut 10 is tightened over the bolt portion 9a of the core 9. The mold 2 is then inserted through the hole 6a into the pressure tube 5, the clamp 12 is then threaded to the retainer case 6 in opening 6e, whereupon the preparatory process is completed. Then, the pressure chamber 8 defined between the retainer case 6 and the pressure tube 5 is supplied with a liquid 7 under pressure through the liquid supply and discharge ports 6c, 6d. As the liquid 7 under pressure is supplied, the flexible pressure tube 5 is pressed against the substantially the entire outer surface 2a of the mold 2 simultaneously, although not specifically shown. Under the radially inward pressure from the pressure tube 5, the flexible mold 2 has its inside diameter D reduced to simultaneously pressurize the entire column of powder 14 in the powder filling space 1. The pressure of air present in the powder 14 is increased as the pressure of the liquid 7 increases. Therefore, the air passes through minute air passages between the powder particles toward the lids 3, 4, and is discharged through the thread gaps between the bolt portions 9a, 9b and the nuts 10, 11. After the pressurization is performed over a prescribed time, the pressure liquid 7 is discharged from the pressure chamber 8 through the liquid supply and discharge ports 6c, 6d. As the pressure of the liquid 7 is reduced, the flexible mold 2 and the pressure tube 5 resiliently return to their original shape until the original inside diameter D of the mold 2 is reached. Then, the upper clamp 12 is detached from the retainer case 6 and the mold 2 is removed from the retainer case 6. Finally, the formed product (not shown) in the mold 2 and the mold 2 and the core 9 are separated from each other.
As described above, the pressure of air in the powder 14 is increased as the pressure of the liquid 7 is increased, and the air flows through the air passages between the powder particles toward the lids 3, 4 and is discharged out through the gaps between the bolt portions 9a, 9b and the nuts 10, 11. In order to discharge the air in the powder 14 quickly, it is necessary that the air pressure be high and the air passages defined as continuous gaps between the powder particles be large. To increase the air pressure, the pressure of the liquid 7 should be increased. As the powder 14 is pressurized, however, the air passages defined as continuous gaps between the powder particles are greatly reduced or closed. Accordingly, increasing the air pressure and enlarging the air passages are contradictory to each other.
In the conventional dry-type rubber pressing method and apparatus, as the liquid 7 under pressure is supplied, the overall pressurizing region in the mold 2 is substantially simultaneously pressed to reduce the inside diameter D of the mold 2. As a consequence, the pressure of the air in the powder 14 is increased and at the same time the gaps between the powder particles are reduced or closed throughout the entire powder filling space 1. Therefore, the above contradictory problem cannot be resolved, and the following drawbacks are caused: Where the longitudinal dimension H of the powder filling space 1 is increased, the air compressed under high pressure in the powder 14 filled centrally in the powder filling space 1 cannot be removed completely since the distance to the air-discharging gaps defined between the bolt portions 9a, 9b and the nuts 10, 11 is large, with the result that compressed air remains trapped in the formed product. The air trapped in the formed product tends to be expanded to damage the product when the pressure of the liquid 7 is reduced. To prevent compressed air from remaining in the product, it has been necessary for the longitudinal dimension H of the powder filling space 1 in the conventional dry-type rubber pressing method and apparatus to be 500 mm or less. This is disadvantageous in that longer products cannot be produced.